Taxonomy & naming
Melanosternarchus amaru was described in 2018 by Bernt, Crampton, Orfinger and Albert in Zootaxa, based on a holotype (MUSM 59405, 9 in total length) collected from the Rio Nanay at Pampachica, in the Maynas district of Loreto, Peru. It belongs to the family Apteronotidae, the ghost knifefishes, within the order Gymnotiformes — the New World electric knifefishes, a group of bony fish defined by their electric organs and their ribbon-like anal fin rather than by any resemblance to other aquarium families. The genus Melanosternarchus was erected in the same paper; phylogenetic analysis placed it as sister to the genus Compsaraia, within the broader Pariosternarchus clade of apteronotids.
The species epithet amaru comes from the Quechua word for a mythical serpent, a fitting name for a long, dark, eel-like fish from the Amazonian rivers of Peru. Despite the common English name "knifefish" that these Gymnotiformes share with the Asian featherback and clown knifefishes (Chitala, Notopterus), the two groups are not related; the similar blade-like body shape is convergent, not a sign of kinship. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the valid name.
Morphology
Melanosternarchus amaru was diagnosed in its original description by an expanded infraorbital laterosensory canal system, a reduced number of premaxillary teeth, a comparatively large gape, and a scaleless dorsum — a combination of features not previously documented together within Apteronotidae. Like other ghost knifefishes it has a laterally compressed, blade-shaped body, no dorsal fin, and no conventional tail fin; locomotion comes from a single long anal fin run as a travelling wave beneath the body, which lets the fish move forward or backward with equal ease. The holotype measured 9 in total length, and a paratype (FMNH 128388) measured 8.5 in TL, giving a working sense of adult size for the species.
As in all Apteronotidae, an electric organ built from modified nerve tissue runs along the body and drives a fast, wave-type electric organ discharge, paired with sensory pores across the skin that read the resulting field — the species' primary means of sensing its surroundings in the dark, sediment-laden channels it inhabits. One collected specimen was observed to have consumed the tail of another electric fish, a behaviour noted as unusual for the family; around 40% of specimens collected from the Rio Nanay showed tail damage or regeneration, hinting at either predation pressure or intraspecific interaction, though this is not well understood. The species is scaleless, as is typical of the order.
Habitat
Melanosternarchus amaru is known from the deep channels of blackwater and clearwater tributaries of the Amazon basin in Peru, with its type locality in the Rio Nanay near Iquitos, Loreto. Its original description noted a preference for low-conductivity blackwater and clearwater systems that had not previously been documented within Apteronotidae, distinguishing its habitat use from many of its relatives.
These are warm, dim, structurally complex river channels — the kind of water where sight is of limited use and an electric sense does the work instead. Beyond the collection localities recorded in its description, little has been published on the species' broader habitat preferences, seasonal movement, or depth range; as a deep-channel fish rarely encountered outside targeted ichthyological survey, much of its natural history remains undocumented.
Feeding
No dedicated dietary study of Melanosternarchus amaru has been published. Based on its Apteronotidae relatives and its morphology — a large gape and reduced dentition noted in the original description — it is reasonable to infer a nocturnal, carnivorous diet of small benthic invertebrates, though this has not been confirmed by gut-content analysis or field observation for this species specifically.
As with other ghost knifefishes, prey detection in the wild would be expected to rely heavily on electrolocation, using the electric field generated by the fish to sense the electrical signature of prey hidden in sediment or among cover in dark water. Data on this species' actual feeding behaviour is sparse.
Mating
No courtship or mating behaviour has been documented for Melanosternarchus amaru. In ghost knifefishes generally, the electric organ discharge functions as a social signal alongside its sensory role, carrying information about identity, sex and readiness to spawn; in well-studied relatives such as the brown ghost (Apteronotus leptorhynchus), males and females shift their discharge frequency during courtship in a kind of electrical exchange. Whether M. amaru shows comparable signalling behaviour has not been studied, and nothing about its courtship or spawning approach has been reported in the literature.
Breeding
Breeding biology in Melanosternarchus amaru is undocumented. Apteronotidae as a family are egg-layers, typically depositing eggs among cover such as roots or crevices with no substantial parental care, but this species has no aquarium history and no field observations of spawning have been published. Data sparse; nothing specific to this species can be responsibly stated beyond the general pattern for its family.
In the aquarium
Melanosternarchus amaru has essentially no presence in the aquarium hobby. It was described from scientific collections rather than trade specimens, and there is no record of it being kept, bred, or offered for sale. Any care guidance can only be extrapolated from its family: Apteronotidae are scaleless and correspondingly sensitive to ammonia, nitrate and to copper-based or harsh medications, which would need to be dosed with caution should the species ever appear in the trade. As a deep-channel, nocturnal predator adapted to dim, low-conductivity water, it would be expected to need generous cover, subdued lighting and warm, soft, clean water — but none of this has been tested in captivity, and the species should be treated as effectively unavailable and unkept rather than as a known aquarium fish.
Conservation
Melanosternarchus amaru has not been formally assessed by the IUCN Red List; no specific status is available, and the species is best treated as Data Deficient given how recently it was described and how little is known of its population size, range extent or trends. It is known from a small number of collected specimens from the Rio Nanay and nearby Amazonian tributaries in Loreto, Peru, and has no documented presence in the aquarium trade to raise collection pressure.
As a narrow-range, deep-channel endemic of Peruvian Amazon tributaries, the species would be expected to face the same general pressures as other blackwater and clearwater fish of the region — habitat change, deforestation, and water-quality impacts from mining and development in the Loreto region — but no targeted conservation assessment or monitoring specific to M. amaru has been published.